1. Field of the Invention
The present invention is directed to innovations in the manufacture of rock bits. More particularly, the present invention is directed to cast steel rock bit cutter cones into which hard cermet cutting inserts are incorporated during the casting process.
2. Brief Description of the Prior Art
Rock bit cutter cones having cemented carbide-type cutter inserts are, generally speaking, used for drilling in subterranean formations under conditions where other drilling cones, such as "milled tooth" cones, would provide relatively low rates of penetration and shorter bit runs. The hard cutter inserts incorporated into rock bits typically comprise cermets, such as tungsten carbide (or other hard metal carbide) in a metal binder phase. The most frequently used cutter inserts for rock bits comprise tungsten carbide in a cobalt binder (WC-Co).
In accordance with typical prior art practice for the preparation of cutter cones having cermet inserts, the steel cutter cones are made first by forging. Thereafter, holes are drilled into the steel cutter cone for accepting the cermet cutter inserts. The cutter inserts usually have a cylindrical base and are usually mounted into the holes with an interference fit. This method of mounting the cutter inserts to the cone is not entirely satisfactory, however, because it is labor intensive. Moreover, the inserts are often dislodged and lost from the cone due to excessive forces, repetitive loads, and shocks which unavoidably occur during subterranean drilling.
With regard to the foregoing, it should be recognized by those skilled in the art that retention of the inserts in the cone is highly dependent on the yield strength of the cone materials. However, in conventional cones, it is not possible or practical to increase the retention beyond a certain upper limit because increasing yield strength usually results in lowered fracture toughness potentially leading to cone cracking in service. Therefore, the acceptable upper limit of the yield strength of the cone is limited by the fracture toughness of such material and therefore rock bit insert retention through interference techniques is consequently limited.
In light of the foregoing and in an effort to improve the attachment of the cutter inserts to the cutter cones, the prior art has devised several techniques. For example, U.S. Pat. No. 4,389,074 describes brazing tungsten carbide cobalt inserts into a mining tool with a brazing alloy. U.S. Pat. No. 3,294,186 describes mounting of tungsten carbide cobalt inserts into rock bits using a layer of a brazing alloy, a nickel shim, and yet another layer of a brazing alloy. The procedure described in these two patents, however, is very labor intensive, because the brazing is performed in connection with each insert after the cutter cone, having the appropriate apertures for the inserts, has already been formed by conventional techniques.
Another approach taken by the prior art to improve the mounting of cutter inserts to the cutter cones, is to provide a widened, reverse taper base for the cutter inserts. Such inserts are mounted into the cutter cones by embedding the insert in a suitable metal powder, and thereafter forming the cutter cone through powder metallurgy processes.
A significantly improved rock bit cutter cone, having strongly bonded cutter inserts, is described in U.S. patent application Ser. No. 544,923, filed on Oct. 24, 1983 which is assigned to the same assignee as the present application and is now abandoned. The cutter cone of the invention described in the above-noted application has a steel core covered by a hard cladding formed by a suitable powder metallurgy process. Hard cermet cutter inserts are mounted into holes or openings provided in the steel core. The inserts are metallurgically bonded to the core and cladding during the hot isostatic pressing or like process in which the cladding is consolidated.
Still other techniques for affixing tungsten carbide inserts to drill bodies, tools, and the like are described in U.S. Pat. Nos. 1,926,770 and 3,970,158.
A problem encountered in the prior art in connection with cermet cutter inserts, and particularly tungsten carbide cobalt (WC-Co) cutter inserts relates to the formation, under certain conditions, of undesirable metallurgical phases, such as a brittle "eta" phase, in the WC-Co cutter inserts. More specifically, when the cermet insert surrounded by steel, such as a WC-Co insert mounted into a steel rock bit cutter cone, is heated to high temperature, the above-noted "eta" phase is formed in the insert, and the toughness and durability of the insert deteriorates significantly.
As is well understood by those skilled in the metallurgical sciences, the "eta" phase is formed in the tungsten carbide cobalt insert by Fick's Law diffusion of carbon from the insert into the surrounding steel cone matrix. Essentially, the relatively high carbon content of the tungsten carbide cobalt insert, and the high affinity of the adjacent steel for carbon, provide the driving force for the above-noted diffusion, and cause the attendant deterioration of the insert.
Except for the above-mentioned application for U.S. patent Ser. No. 544,923, the prior art was, by-and-large, unable to prevent the formation of undesirable "eta" phase in WC-Co cutter inserts under the above-noted conditions. The foregoing provides perhaps the principal reason why, up to the present invention, the majority of rock bit cutter cones which had WC-Co cutter inserts, had the inserts merely interference fitted in the apertures previously formed in the steel cone of the rock bit.
Moreover, even though it has been considered desirable to have a thermal barrier on the insert for minimizing or eliminating thermally generated fracture associated with casting, as well as retarding or eliminating "eta" phase formation, the prior art was limited in this regard to titanium nitride and titanium carbide coated inserts. The titanium nitride and titanium carbide coated inserts, however, are not bonded to the resulting steel matrix by metallurgical bonds. Therefore, often they are held loosely, and under harsh conditions are likely to rotate, to be lost, or to initiate cracking in the steel matrix.